This invention relates to an image sensor and more particularly to an image sensor comprising an array having control circuitry integrated onto the same substrate as image sensing pixels of the array.
Large area integrated circuit image sensor arrays are known using diode-based pixel arrangements. For example, EP-0-154 997 discloses an image sensor in which each image sensing pixel comprises a photodiode connected in series with a switching diode, the two diodes being connected with opposite polarity.
It is desired to reduce the dimensions of the pixels of image sensing arrays so as to increase the image sensing resolution. For example, some applications require a resolution of 600 dpi (dots per inch) which corresponds to a maximum pixel dimension of 42 xcexcm (micrometers.) Conventional pixel arrangements which utilise a switching diode and a photodiode are arranged so that these components are positioned adjacent one another, such that layers deposited on the substrate are shared between the two diode devices. This gives rise to the problem that the two diode devices must necessarily have the same construction, and the requirement for each pixel to include two side by side diodes limits the possible reduction in size of the pixel.
The pixel arrangement of EP-0-154 997 provides a stacked arrangement with the photodiode and the switching diode disposed one above the other. This has the advantage that a further reduction in the size of the pixel is possible and the layers used for the switching diode and for the photodiode can be optimised for each respective diode.
A further problem encountered with the prior art is that the large number of pixels give rise to a very high number of connections to the array of image sensor pixels. Various multiplexer circuits have been proposed which may be integrated onto the substrate of the image sensor array, so as to reduce the number of connections which must be made to the array. Of course, the multiplexer circuits, or other control circuitry to be integrated onto the substrate of the image sensor pixels, must utilise device elements corresponding to the elements of the image sensor pixels.
The image sensor pixels of EP-0-154 997 comprise a pair of back to back diodes, and this document does not address the problems associated with the control and interfacing of the array.
The present invention aims to provide a pixel arrangement which allows a reduction in the pixel size, and hence an increased image sensor resolution, and which has control circuitry integrated onto the substrate of the image sensor array.
According to the present invention, there is provided an image sensor comprising an array having an imaging area comprising a plurality of image sensing pixels, and control circuitry comprising a plurality of elemental devices, the image sensing pixels and elemental devices being provided on a common substrate and being defined by layers provided on the substrate, each pixel and each elemental device comprising a diode stack, of a photodiode and a switching diode connected in series with opposite polarity and disposed one above the other with respect to the substrate, the image sensor further comprising illumination means for illuminating the photodiodes of the elemental devices of the control circuitry.
In the image sensor of the invention, the stacked diode pixel configuration enables the pixel dimensions to be reduced and enables the layers provided on the substrate to be optimised with respect to the devices being produced. The illumination of the photodiodes of the control circuitry enables the back to back diode arrangement to be used as circuit elements. In particular, this illumination of the photodiodes has the effect of operating each photodiode as a constant current source. This enables the switching diodes to be used as normal switching elements, and the effect of the constant current source in series with the switching diode is to provide limitation of the peak current flowing through the switching diode.
Preferably, the photodiode and switching diode comprise amorphous silicon devices, and the diode stack preferably comprises a p-i-n-i-p structure, one of the switching diode and photodiode being a p-i-n device, and the other being an n-i-p device. However, other combinations of diode configurations may be used.
Preferably, a light shielding layer is provided within each diode stack between the photodiode and the switching diode. This prevents the passage of light not absorbed by the photodiode into the switching diode structure.
The photodiode may be disposed above the switching diode with respect to the substrate, and in this case the illumination means provides light to the image sensor array from the side of the substrate opposite to the layers provided on the substrate, and reflection means is provided over the control circuitry for reflecting the light to the photodiodes of the elemental devices.
Alternatively, the photodiode may be disposed below the switching diode with respect to the substrate, and in this case the layers provided on the substrate are preferably disposed over a conducting contact portion, each contact portion including an opening which allows the passage of light to the photodiode of the respective diode stack.
When the image sensor comprises a document imaging system the illumination means preferably comprises a light source of the image sensor for illuminating the document to be imaged, so that the image sensor requires only a single light source arrangement. However, the control circuitry may be illuminated separately from the imaging light source of the sensor array.
The pixels are preferably arranged in rows and columns, and the control circuitry may comprise row driver circuitry and/or column reading circuitry.